On Sun, Nov 17, 2013 at 7:47 PM, Robert Krahn <robert.krahn@gmail.com> wrote:

instance Tester Integer where
test 0 = False
test _ = True

For the Maybe instance I want to delegate to the value of Just and add a class constraint:

instance (Tester m) => Tester (Maybe m) where
test Nothing = False
test (Just x) = test x

It compiles nicely and works for Just values

test (Just 3) -- True
test (Just 0) -- False

But
test Nothing

gives me
No instance for (Tester a0) arising from a use of `test'

Several things are happening here. First is that numbers undergo defaulting; with nothing else to specify the type of `3` or `0`, they default to Integer and all is well.

This cannot be said of `Nothing`; it needs to pick an instance, but `Nothing` doesn't give it anything to work from.

"But I only have an instance for Integer!" Typeclasses work under the "open world assumption": a typeclass could be added at any time, so it cannot commit to a given typeclass instance simply because it only happens to know of one valid instance. In particular, it cannot conclude that `Nothing` is of type `Maybe Integer` solely because it happens to only be aware of a `Tester Integer` instance.

(Instances are program global and cannot be hidden, so not making the open world assumption could easily cause programs to completely change their meaning with the addition of a single additional instance *anywhere* in that program, even if not imported to that module.)

brandon s allbery kf8nh sine nomine associates

allbery.b@gmail.com ballbery@sinenomine.net

unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net